Arsenic Removal Fiom Geothermal Bore Water: the Effect of Dissolved Silica Rotorua

Abstract

Arsenic adsorption onto a hydrous ferric oxide (HFO) floc is a viable option for the removal of arsenic (As) from geothermal bore waters. However, the degree of As@) and A s 0 adsorption h m bore water is considerably less than that hm a O.lm NaN03 electrolyte, or than that predicted using equilibrium swface complexation constants and the geochemical speciation model, MINTEQM. By determining the effect of individual bore water components on the adsorption of A s 0 and A s 0 hm O.lm NaN03, mono-silicic acid fiSi04) was identified as the main component inhibiting As adsorption. Mono-silicic acid is adsorbed onto the HFO surface when the &SiO#e ratio of the solid is below ca. 0.2, and polymerised on the surface at higher &SiO#e ratios. Both of these processes appear to inlibit As adsorption from bore water. By altering the MINTEQA2 data base to include a surface complexation constant for 114Si04 adsorption onto HFO, prediction accuracy is improved. However, MINTEQM continues to overestimate the degree of As(II1) adsorption, and of As(V) adsorption at pH > 8. Tlis is considered to reflect the effect of 114Si04 polymerisation on the HFO surface, a process which is not taken into account by the speciation model.